Experimental autoimmune encephalomyelitis (EAE) is an animal model that reproduces many of the clinical and pathological features of multiple sclerosis (MS). The development and the progression of EAE, like other autoimmune diseases, results from the pathogenicity of myelin specific effector T cells and the negative regulation imposed by regulatory T cells (Tregs). Of all the encephalitogenic effector T cells, in recent years IL- 17-producing Th17 cells have been shown to be most important in mediating tissue inflammation and autoimmunity in multiple autoimmune diseases, including EAE. However, not all Th17 cells are pathogenic. In fact, we have identified both pathogenic and nonpathogenic Th17 cells. Identifying the factors that make Th17 cells pathogenic is crucial for achieving the selective inhibition of pathogenic Th17 cells while sparing the nonpathogenic Th17 cells, which have an important role in maintaining barrier function at mucosal surfaces. We have identified that IL-23 is a critical cytokine in the process, which endows developing Th17 cells with a pathogenic phenotype and the ability to induce autoimmunity. To address how IL-23 induces pathogenic Th17 cells, we have undertaken a systematic temporal microarray analysis at 18 different time points and developed a transcriptional network to identify signaling molecules and factors that are responsible for the induction of pathogenic Th17 cells. We have identified a kinase, serum glucocorticoid kinase 1 (SGK-1), downstream of IL- 23R that is critical for inducing pathogenic Th17 cells. More recently, we have undertaken a single cell RNA- seq analysis of differentiating Th17 cells and identified CD5L (CD5L like) as a key molecule that is expressed in nonpathogenic Th17 cells and whose expression is downregulated by IL-23R signaling. We hypothesize that SGK-1 kinase activated by IL-23R signaling induces pathogenic Th17 cells not only by activating transcriptional modules that make Th17 cells pathogenic but also by inhibiting molecules like CD5L, which make Th17 cells nonpathogenic. In addition, our preliminary data suggest that SGK-1 also suppresses the generation of Foxp3+ Tregs. We hypothesize that SGK-1 is a pivotal node, induced and activated by IL-23R signaling, that promotes the generation of pathogenic Th17 cells, suppresses inhibitory molecules like CD5L that make Th17 cells nonpathogenic, and concomitantly inhibits FoxP3+ Treg development. In this grant we propose two specific aims that will determine the mechanism by which: 1) SGK-1 induced by IL-23R signaling regulates the balance between Th17 and FoxP3+ Tregs; 2) CD5L, which is expressed on nonpathogenic Th17 cells, regulates the development of nonpathogenic Th17 cells. These studies will deepen our understanding of the cellular and molecular basis for the induction of pathogenic TH17 responses and, more importantly, will inform as to how we can selectively suppress pathogenic TH17 cells but spare beneficial TH17 cells.